One common method of manufacturing contact lenses is lathing the lens from a cylindrical contact lens button. Contact lens buttons may be cut from rod stock or sheet material, or individually cast in cups using a curable liquid monomer. While it is possible to attach the button directly to the lathe chuck, this is not commonly done when cutting the front curve of the lens since the chuck of the lathe machine uses radial compression to hold the work piece in place for lathing, and as such could impart damaging forces to the optical characteristics of the button. The base curve, on the other hand, may be cut by chucking the button or by adhesively mounting the button as described below.
For lathing the front curve and/or the base curve, the button is commonly mounted to one end of a separate metal pin, also referred to as a “block” or “mandrel”, the opposite end of the block being configured as a spindle for removable insertion into the chuck of the lathe. The button is typically adhered to the block with an adhesive or special type of wax called “blocking wax” in the art. While the block and button are turned, a concave (or “base”) curve is lathed into the exposed end of the button opposite the adhered end. While still attached to the chuck and rotating, a second “ledge” cut may be made in the outer cylindrical surface of the button for facilitating subsequent alignment with the front curve block, this cut typically termed a “base curve reference”. A “front curve” block is then removably secured to the lathed base curve surface while attempting to maintain axial alignment with the first “base curve” block which is then removed from the button. The front curve block is mounted to the lathe chuck for lathing the convex or front curve of the button to form the finished lens. Lens edging and polishing operations may also be performed on each surface of the lens prior to removal from each block.
During machining, the lens lathe cutter head applies a normal force (perpendicular to the plane of the machined surface) to the surface of the lens blank. In response to this normal force, the lens blank elastically deforms. If the amount of elastic deformation were consistent among the blanks, then the lathe operator could in theory adjust the lathe settings to reliably account for this deformation. Thus it would be desirable for the extent of elastic deformation to be consistent within batches of contact lens buttons. If a correction for elastic deformation must be factored into the machining algorithms at all, it would be desirable for that correction to be as small as possible to minimize the potential for error. Thus it would be desirable to minimize elastic deformation of the contact lens button during machining.
Conventional blocking waxes are deformable adhesives. One example of a useful blocking wax for lathing rigid gas-permeable (RGP) contact lenses Kerr Optical Soluble wax, 6502-A from Universal Photonics of Hicksville, N.Y.
The conventional way of machining an RGP contact lens is to apply a small amount of blocking wax either to the end of a mandrel or to the RGP lens button, and then to press the RGP button onto the mandrel. Mounting and removing operations of the button to and from the blocks during the entire lathing process consumes a significant amount of time and is prone to error, especially in maintaining the necessary axial alignment between the base curve block and the front curve block. If the base and front curve blocks are not aligned precisely, an undesired prism may be introduced to the resultant lens that must then be scrapped.
The conventional ways of removing the lens button from the mandrel include mechanically prying the button off of the mandrel, or heating the mandrel so that the blocking wax melts. These extra steps make the manufacturing process more expensive.
Thus it would be desirable to formulate a blocking wax composition that provides a stable adhesive base to firmly hold the lens blank to the mandrel. It would also be desirable to provide a blocking wax composition that both reduces the extent of elastic deformation in the lens blank during machining and makes that elastic deformation more predictable. Finally, it would be desirable to provide a blocking wax composition that could easily be removed from the lens button and the lathe mandrel after the machining operations are complete.